B. glabrata, when infected with x-irradiated miracidia of certain species of trematodes, acquires specific resistance to a subsequent challenge with non-irradiated miracidia of the same species. In addition, preinfection with nonschisotosome miracidia interfers with the active resistance of the snails to S. mansoni, rendering them temporarily susceptible. These phenomena are being studied with the trematodes Echinostoma paraensei and Ribeiroia marini. The ability of three stocks of B. glabrata to develop resistance to the 4 most commonly used molluscicides is being compared. Successive generations of survivors of molluscicide exposures are being reared and tested for molluscicide resistance.